Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Chapter 10: Mass-Storage Systems

10.2 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Chapter 10: Mass-Storage Systems Overview of Mass Storage Structure Disk Scheduling

10.3 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Objectives To describe the physical structure of secondary storage devices and its effects on the uses of the devices To explain the performance characteristics of mass-storage devices To evaluate disk scheduling algorithms

10.4 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Moving-head Disk Mechanism

10.5 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Overview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 250 times per second Transfer rate is rate at which data flow between drive and computer Positioning time (access time) = seek time + rotational latency seek time: time to move disk arm to desired cylinder rotational latency: time for desired sector to rotate under the disk head Head crash results from disk head making contact with the disk surface Drive attached to computer via I/O bus Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI, SAS, Firewire

10.6 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Hard Disks Platters Commonly 3.5”, 2.5”, and 1.8” Size 30GB to 3TB per drive Performance Transfer Rate  6 GB/sec in Theory and 1GB/sec in practice Seek time  3ms to 12ms – 9ms common for desktop drives Rotation Latency based on spindle speed (From Wikipedia)

10.7 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Solid-State Disks Nonvolatile memory used like a hard drive Can be more reliable than HDDs More expensive per MB Maybe have shorter life span Less capacity Much faster No moving parts, so no seek time or rotational latency

10.8 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Magnetic Tape Mainly used for backup Holds large quantities of data 200GB to 1.5TB typical storage Access time slow Random access ~1000 times slower than disk Once data under head, transfer rates comparable to disk 140MB/sec and greater

10.9 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Disk Scheduling The operating system is responsible for using hardware efficiently Minimize access time

10.10 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Disk Scheduling (Cont.) I/O request includes input or output mode disk address memory address number of sectors to transfer OS maintains queue of requests, per disk or device Idle disk can immediately work on I/O request Busy disk must queue requests Optimization algorithms apply

10.11 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Disk Scheduling (Cont.) Several algorithms exist to schedule the servicing of disk I/O requests We illustrate scheduling algorithms with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53

10.12 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition FCFS Illustration shows total head movement of 640 cylinders

10.13 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition SSTF Shortest Seek Time First selects the request with the minimum seek time from the current head position form of SJF scheduling; may cause starvation of some requests Illustration shows total head movement of 236 cylinders

10.14 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition SCAN The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues. SCAN algorithm Sometimes called the elevator algorithm Illustration shows total head movement of 208 cylinders But note that if requests are uniformly dense, the largest density is at other end of disk and thus they will wait the longest Sol: C-SCAN

10.15 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition SCAN (Cont.)

10.16 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition C-SCAN Provides a more uniform wait time than SCAN The head moves from one end of the disk to the other, servicing requests as it goes When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip Treats the cylinders as a circular list that wraps around from the last cylinder to the first one Total number of cylinders?

10.17 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition C-SCAN (Cont.)

10.18 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition C-LOOK LOOK a version of SCAN C-LOOK a version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk Total number of cylinders?

10.19 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition C-LOOK (Cont.)

10.20 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition Selecting a Disk-Scheduling Algorithm SSTF is common and has a natural appeal SCAN and C-SCAN perform better for systems that place a heavy load on the disk Less starvation The disk-scheduling algorithm should be written as a separate module of the operating system, allowing it to be replaced with a different algorithm if necessary

Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9 th Edition End of Chapter 10